An anode of a lithium secondary battery comprises a current collector (for example, copper foil) and an active material layer (for example, carbon, tin alloy and the like) laminated on the current collector. In this lithium secondary battery, expansion/contraction of the active material layer occurs since lithium enters and exits the active material layer during charge and discharge. In addition, the active material layer gets broken due to the stress produced by the expansion/contraction. As a result, the charge and discharge capacity of the lithium secondary battery decreases after many repeats of charge and discharge cycle.
Patent document 1 describes configuration of a void (opening) with a given pattern in an active material layer to alleviate the above-mentioned stress. As specific shape of the active material layer, a stripe-shape (shape of opening: stripe-shape), a grid-shape (shape of opening: square) and a square dot-shape (shape of opening: grid-shape) are described (particularly, patent document 1, FIG. 3).
In patent document 2, configuration of a void (opening) in a current collector itself in an attempt to alleviate the above-mentioned stress and prevent fracture of a current collector is described (particularly, patent document 2, FIG. 1).